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Summary of significant accounting policies - Estimated Useful Lives of Related Assets (Details)
12 Months Ended
Dec. 31, 2015
Minimum [Member] | Premises and Leasehold Improvements [Member]  
Property, Plant and Equipment [Line Items]  
Property, plant and equipment, estimated useful life 7 years
Minimum [Member] | Furniture, Fixtures and Equipment [Member]  
Property, Plant and Equipment [Line Items]  
Property, plant and equipment, estimated useful life 3 years
Maximum [Member] | Premises and Leasehold Improvements [Member]  
Property, Plant and Equipment [Line Items]  
Property, plant and equipment, estimated useful life 40 years
Maximum [Member] | Furniture, Fixtures and Equipment [Member]  
Property, Plant and Equipment [Line Items]  
Property, plant and equipment, estimated useful life 10 years